(1) Field of the Invention
This invention relates to an optical material, and specifically, to an optical material having superb practical properties typified by excellent dyeability and high refractive index.
(2) Description of the Related Art
Inorganic glass has heretofore been used widely as an optical material for optical parts or members of various precision machines and instruments, camera lenses, spectacle lenses, dark glasses, etc. Optical materials composed of plastics have however been finding utility in recent years as substitutes for inorganic glass from the viewpoint of weight reduction.
In addition to their merit of light weights, plastic-made optical materials have another attractive property in that they can be dyed even after their molding or forming unlike inorganic glass. This dyeability is important especially in the case of spectacle lenses.
As plastics which have heretofore been used abundantly as plastic-made optical materials, polymethyl methacrylate, polystyrene, polycarbonates and polydiethylene glycol bisallyl carbonate called "CR-39" (trade name; product of PPG Industries, Inc.) are known.
Polystyrene however does not permit its dyeing at all with any water-dispersible dye after its molding or forming. In some instances, a mere contact to an organic solvent may result in a detriment attack to its surface so that cracks may be formed there. Moreover, polystyrene is insufficient in heat resistance. It is hence a material having poor practical utility as an optical material.
On the other hand, polymethyl methacrylate and polycarbonates are dyeable with a water-dispersible dye even after their molding or forming. These materials however involve such problems that in addition to their susceptibility to an attack by an organic solvent, polymethyl methacrylate does not have sufficient heat resistance while polycarbonates have a large optical strain, and a significant limitation has thus been imposed on their utility as optical materials.
Although "CR-39" does not have injection moldability, it may generally be regarded as a preferable material in that it has a crosslinked structure of a sufficiently high degree, is equipped with good heat resistance and solvent resistance sufficient to withstand ordinary organic solvents and moreover, has dyeability. This optical material is however is accompanied by a drawback that its refractive index is as low as 1.49 (n.sub.d =1.49). If a spectacle lens should be formed with this material, the lens thickness will become significantly greater compared to that required for a material having a high refractive index so that its light-weight merit will be lost as a matter of fact.
Under the circumstances, attempts have been made to increase the refractive index of "CR-39" while retaining its advantageous properties. Specifically, investigations are now under way with respect to copolymerization of diethylene glycol bis(allyl carbonate) with a monomer having a high refractive index.
The radically-polymerizable functional groups of diethylene glycol bis(allyl carbonate) are however allyl groups, which have lower radical polymerizability compared to polymerizable groups of many monomers, such as acryl groups.
Accordingly, monomers capable of undergoing suitable copolymerization with diethylene glycol bis(allyl carbonate) are limited to those containing one or more allyl groups as functional groups in practice. It is difficult to copolymerize it with many monomers which contain one or more useful functional groups having high radical polymerizability, such as acryl, methacryl and/or vinyl groups. For these reasons, it is the current situation that no copolymer has been provided with a high refractive index while retaining the excellent physical properties of "CR-39".
From the foregoing circumstances, it is desired to provide an optical material which has excellent properties such that it permits easy copolymerization with monomers having one or more functional groups of high radical polymerizability, such as acryl, methacryl and/or vinyl groups, has a crosslinked structure of a sufficiently high degree and moreover, possesses good dyeability.
As has already been mentioned above, there is an outstanding demand for the development of optical materials having a high refractive index. A variety of proposals have been made toward this demand, and some of them have already been used actually. It has been known to be effective to have an aromatic compound or a halogen-substituted aromatic compound included as a component in order to obtain an optical material having a high refractive index. A variety of investigations have been made making use of this technique. Introduction of such a component into a plastic-made optical material however leads to an optical material of reduced impact resistance or of a greater specific gravity, so that the inherent merits of the plastic-made optical material are sacrificed. As a matter of fact, it has not been succeeded to provide any optical material capable of affording a plastic lens having a high refractive index, great impact resistance and a small specific gravity.